Microtus montanus, shifts from nocturnal activity under long photoperiod to diurnal activity under short photoperiod. This activity change is mediated in males by photoperiodically-induced changes in gonadal function in this seasonally breeding rodent. High gonadal function (or testosterone (T) administration) increases nocturnal activity regardless of photoperiod. Castration results in diurnal or crepuscular (dawn and dusk activity) regardless of photoperiod. This response of the Microtus circadian system is far more dramatic than those documented for other species response to hormonal manipulations. The aim of this proposal is to use the male M. montanus response to T as a model system for studying the role of reproductive hormone feedback on the circadian pacemaker(s). Since T can be converted to either dihydrotestosterone or 17beta-estradiol, these latter compounds will be tested to determine which of the three actually binds to receptor sites. Experiments will be performed to determine whether T must be administered at a particular phase of the light: dark cycle in order to induce nocturnal activity. If this is the case, then there may be circadian rhythm in the presence of the appropriate receptor sites. Animals with lesions of the suprachiasmatic nuclei will be studied to determine if responses to T can occur in the absence of this important rhythm generator. An experiment will also be performed to see if T administration can entrain activity under freerunning conditions, i.e., in the absence of any other environmental cues. The Microtus system should prove to be an excellent new model system for dealing with questions of the integration and hierarchies of circadian rhythms for two reasons: 1) the dramatic response to T, 2) the fact that the same species can be studied as either nocturnal or diurnal. Circadian rhythmicity is the foundation upon which the complex system of reproductive function is built. Increasing our knowledge of the integration of circadian rhythms will provide better understanding of reproductive biology.